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metadata annotations YANG extension This document defines a YANG extension that allows for defining metadata annotations in YANG modules. The document also specifies XML and JSON encoding of annotations and other rules for annotating instances of YANG data nodes.

There is a need to be able to annotate instances of YANG  data nodes with metadata. Typical use cases are as follows: Complementing regular data model information with instance&nbhy;specific metadata, comments, etc. Providing information about the rendering of data in user interfaces. Deactivating a subtree in a configuration datastore while keeping the data in place. Network management protocols often use metadata annotations for various purposes in both operation requests and responses. For example, the <edit-config> operation in the Network Configuration Protocol (NETCONF) (see Section 7.2 of ) uses annotations in the form of XML attributes for identifying the location in a configuration datastore and the type of the operation. However, metadata annotations could potentially lead to interoperability problems if they are used in an ad hoc fashion by different parties and/or without proper documentation. A sound metadata framework for YANG should therefore satisfy these requirements: The set of annotations must be extensible in a decentralized manner so as to allow for defining new annotations without running the risk of collisions with annotations defined and used by others. The syntax and semantics of annotations must be documented, and the documentation must be easily accessible. Clients of network management protocols such as NETCONF  or RESTCONF  must be able to discover all annotations supported by a given server and identify each of them correctly. Annotations sent by a server should not break clients that don't support them. This document proposes a systematic way to define metadata annotations. For this purpose, the YANG extension "annotation" is defined in the module "ietf-yang-metadata" (). Other YANG modules importing this module can use the "annotation" statement for defining one or more annotations. The benefits of defining the metadata annotations in a YANG module are the following: Each annotation is bound to a YANG module name and namespace URI. This makes its encoding in instance documents (both XML and JSON) straightforward and consistent with the encoding of YANG data node instances. Annotations defined in IETF Standards Track documents are indirectly registered through IANA in the "YANG Module Names" registry . Annotations are included in the data model. YANG compilers and tools supporting a certain annotation can thus take them into account and modify their behavior accordingly. The semantics of an annotation are defined in the "description" and "reference" statements. An annotation can be declared as conditional by using the "if&nbhy;feature" statement. The type of each annotation is explicitly specified; any YANG built-in or derived type that is available for leaf or leaf-list data nodes may be specified for annotations as well. In the XML encoding, XML attributes are a natural instrument for attaching annotations to data node instances. This document deliberately adopts some restrictions in order to remain compatible with the XML encoding of YANG data node instances and limitations of XML attributes. Specifically, annotations can only be scalar values. annotations cannot be attached to a whole list or leaf-list instance, only to individual list or leaf-list entries. Due to the rules for YANG extensions (see Section 6.3.1 in ), annotation definitions posit relatively weak conformance requirements. The alternative of introducing a new built-in YANG statement for defining annotations was considered, but it was seen as a major change to the language that is inappropriate for YANG 1.1, which was chartered as a maintenance revision. After evaluating real-life usage of metadata annotations, it is conceivable that such a new built-in statement might be added in a future revision of YANG.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

The following terms are defined in : capability client datastore message protocol operation server The following terms are defined in : action anydata anyxml built-in type container data model data node data tree derived type extension leaf leaf-list list module Remote Procedure Call (RPC) input and output The following terms are defined in : attribute document element The following terms are defined in : local name namespace name prefix qualified name The following terms are defined in : array member object primitive type

In the following text, XML element names and YANG extension statements are always written with explicit namespace prefixes that are assumed to be bound to URI references as shown in . Prefix URI Reference elm http://example.org/example-last-modified md urn:ietf:params:xml:ns:yang:ietf-yang-metadata rng http://relaxng.org/ns/structure/1.0

annotation: a single item of metadata that is attached to YANG data node instances. metadata: additional information that complements a data tree. metadata object: an object in JSON encoding that contains all annotations attached to a given data node instance.

Metadata annotations are defined by the YANG extension "md:annotation". This YANG language extension is defined in the module "ietf-yang-metadata" (). Substatements of "md:annotation" are shown in . They are all core YANG statements, and the numbers in the second column refer to the corresponding section in  where each statement is described. substatement section in RFC 7950 cardinality description7.21.30..1 if-feature7.20.20..n reference7.21.40..1 status7.21.20..1 type7.6.31 units7.3.30..1 An annotation carries a single value. The "type" substatement, which MUST be present, takes as an argument the name of an existing built&nbhy;in or derived type, and the value of the annotation MUST match this type. See Section 7.4 of  for details. An annotation can be made conditional by using one or more "if&nbhy;feature" statements; the annotation is then supported only by servers that advertise the corresponding feature. The semantics and usage rules for an annotation SHOULD be fully specified in "description", "reference", and "units" statements. An annotation MUST NOT change the data tree semantics defined by YANG. For example, it is illegal to define and use an annotation that allows for overriding uniqueness of leaf-list entries. The "status" statement can be used exactly as it is used for YANG data nodes. A YANG module containing one or more "md:annotation" statements SHOULD NOT be used for defining data nodes or groupings. Also, derived types, identities, and features SHOULD NOT be defined in such a module unless they are used by the definitions of annotations in that module.

The following module defines the "last-modified" annotation:

module example-last-modified { namespace "http://example.org/example-last-modified"; prefix "elm"; import ietf-yang-types { prefix "yang"; } import ietf-yang-metadata { prefix "md"; } md:annotation last-modified { type yang:date-and-time; description "This annotation contains the date and time when the annotated instance was last modified (or created)."; } }

By advertising a YANG module in which a metadata annotation is defined using the "md:annotation" statement, a server indicates that it is prepared to handle that annotation according to the annotation's definition. That is, an annotation advertised by the server may be attached to an instance of a data node defined in any YANG module that is implemented by the server. Depending on its semantics, an annotation may have an effect only in certain data trees and/or on instances of specific types of data nodes. A client MUST NOT add a specific annotation to data node instances if the server didn't advertise it. Due care has to be exercised when introducing annotations in network management systems in order to avoid interoperability problems and software failures caused by a client that does not understand the annotations' semantics. Generally, it is safe for a server to use annotations in the following cases: An annotation is an integral part of a built-in or negotiated protocol capability. An annotation contains auxiliary information that is not critical for protocol operation. The client explicitly asks the server, e.g., via a parameter of a protocol operation request, to include an annotation in the response.

XML attributes are a natural choice for encoding metadata in XML instance documents. For JSON , there is no generally established method for encoding metadata. This document thus introduces a special encoding method that is consistent with the JSON encoding of YANG data node instances as defined in .

Metadata annotations are added to XML-encoded instances of YANG data nodes as XML attributes according to these rules: The local name of the attribute SHALL be the same as the name of the annotation specified in the argument of the corresponding "md:annotation" statement. The namespace of the attribute SHALL be identified by the URI that appears as the argument of the "namespace" statement in the YANG module where the annotation is defined. It is RECOMMENDED that the prefix specified by the "prefix" statement in the same module be used in the qualified name of the attribute. The attribute value SHALL be encoded in the same way as the value of a YANG leaf instance having the same type; see Section 9 of . For example, the "last-modified" annotation defined in  may be encoded as follows:

<foo xmlns:elm="http://example.org/example-last-modified" elm:last-modified="2015-09-16T10:27:35+02:00"> ... </foo>

The JSON metadata encoding defined in this section has the following properties: The encoding of YANG data node instances as defined in does not change. Namespaces of metadata annotations are encoded in the same way as namespaces of YANG data node instances; see .

All metadata annotations assigned to a YANG data node instance are encoded as members (name/value pairs) of a single JSON object, henceforth denoted as the metadata object. The placement and name of this object depend on the type of the data node as specified in the following subsections. The name of a metadata annotation (as a member of the metadata object) has the following ABNF syntax , where the production for "identifier" is defined in Section 14 of :

annotation-name = identifier ":" identifier

where the left identifier is the name of the YANG module in which the annotation is defined and the identifier on the right is the name of the annotation specified in the argument of the corresponding "md:annotation" statement. Note that unlike member names of YANG data node instances in JSON encoding (see Section 4 in ), for annotations the explicit namespace identifier (module name) must always be present. The value of a metadata annotation SHALL be encoded in exactly the same way as the value of a YANG leaf node having the same type as the annotation; see Section 6 of .

For a data node instance that is encoded as a JSON object (i.e., a container, list entry, or anydata node), the metadata object is added as a new member of that object with the name "@". Examples: "cask" is a container or anydata node:

"cask": { "@": { "example-last-modified:last-modified": "2015-09-16T10:27:35+02:00" }, ... }

"seq" is a list whose key is "name"; annotation "last-modified" is added only to the first entry:

"seq": [ { "@": { "example-last-modified:last-modified": "2015-09-16T10:27:35+02:00" }, "name": "one", ... }, { "name": "two", ... } ]

For an anyxml or leaf instance, the metadata object is added as a sibling name/value pair whose name is the symbol "@" concatenated with the name of the leaf or anyxml member that is being annotated. The namespace part (module name) is included if and only if it is in the name of the annotated member. Examples: "flag" is a leaf node of the "boolean" type defined in module "foo", and we assume that the namespace name has to be expressed in its JSON encoding:

"foo:flag": true, "@foo:flag": { "example-last-modified:last-modified": "2015-09-16T10:27:35+02:00" }

"stuff" is an anyxml node:

"stuff": [1, null, "three"], "@stuff": { "example-last-modified:last-modified": "2015-09-16T10:27:35+02:00" }

For a leaf-list entry, which is represented as a JSON array with values of a primitive type, annotations may be assigned to one or more entries by adding a name/array pair as a sibling of the leaf&nbhy;list entry, where the name is the symbol "@" concatenated with the name of the leaf-list that is being annotated, and the value is a JSON array whose i-th element is the metadata object with annotations assigned to the i-th entry of the leaf-list entry, or null if the i-th entry has no annotations. Trailing null values in that array, i.e., those following the last non-null metadata object, MAY be omitted. For example, in the following leaf-list instance with four entries, the "last-modified" annotation is added to the second and third entries in the following way:

"bibliomod:folio": [6, 3, 7, 8], "@bibliomod:folio": [ null, { "example-last-modified:last-modified": "2015-06-18T17:01:14+02:00" }, { "example-last-modified:last-modified": "2015-09-16T10:27:35+02:00" } ]

defines the standard mapping of YANG data models to Document Schema Definition Languages (DSDL) . This section specifies the mapping for the extension statement "md:annotation" (), which enables validation of XML instance documents containing metadata annotations. The first step of the DSDL mapping procedure, i.e., the transformation of the YANG data model to the hybrid schema (see Section 6 in ), is modified as follows: If the data model contains at least one "md:annotation" statement, then a RELAX NG named pattern definition MUST be added as a child of the root <rng:grammar> element in the hybrid schema. It is RECOMMENDED to use the name "__yang_metadata__" for this named pattern. A reference to the named pattern described in item  MUST be included as a child of every <rng:element> pattern that corresponds to an anydata, container, leaf, leaf-list, or list data node. Every metadata annotation definition in the form

md:annotation ARGUMENT { ... }

is mapped to the following RELAX NG pattern:

<rng:optional> <rng:attribute name="PREFIX:ARGUMENT"> ... </rng:attribute> </rng:optional>

where PREFIX is the prefix bound to the namespace URI of the YANG module that contains the "md:annotation" statement. The above pattern SHALL be inserted as a child of the named pattern described in item . Substatements of "md:annotation" SHALL be mapped to children of the "rng:attribute" pattern exactly as described in Section 10 of . For example, the named pattern (item ), when constructed only for the "last-modified" annotation, will have the following definition:

<rng:define name="__yang_metadata__"> <rng:optional> <rng:attribute name="elm:last-modified"> <rng:ref name="ietf-yang-types__date-and-time"/> </rng:attribute> </rng:optional> </rng:define>

Every "rng:element" pattern that corresponds to an anydata, container, leaf, list, or leaf-list data node will then contain a reference to the above named pattern; for example:

<rng:element name="foo:bar"> <rng:ref name="__yang_metadata__"/> ... </rng:element>

Note that it is not necessary to use such a reference for "rng:element" patterns corresponding to anyxml data nodes because they already permit any XML attributes to be attached to their instances. The second step of the DSDL mapping procedure, i.e., the transformation of the hybrid schema to RELAX NG , Schematron , and Document Semantics Renaming Language (DSRL) schemas, is unaffected by the inclusion of "md:annotation".

<CODE BEGINS> file "ietf-yang-metadata@2016-08-05.yang" module ietf-yang-metadata { namespace "urn:ietf:params:xml:ns:yang:ietf-yang-metadata"; prefix "md"; organization "IETF NETMOD (NETCONF Data Modeling Language) Working Group"; contact "WG Web:   <https://datatracker.ietf.org/wg/netmod/> WG List:  <mailto:netmod@ietf.org> WG Chair: Lou Berger           <mailto:lberger@labn.net> WG Chair: Kent Watsen           <mailto:kwatsen@juniper.net> Editor:   Ladislav Lhotka           <mailto:lhotka@nic.cz>"; description "This YANG module defines an 'extension' statement that allows for defining metadata annotations. Copyright (c) 2016 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info). This version of this YANG module is part of RFC 7952 (http://www.rfc-editor.org/info/rfc7952); see the RFC itself for full legal notices."; revision 2016-08-05 { description "Initial revision."; reference "RFC 7952: Defining and Using Metadata with YANG"; } extension annotation { argument name; description "This extension allows for defining metadata annotations in YANG modules. The 'md:annotation' statement can appear only at the top level of a YANG module or submodule, i.e., it becomes a new alternative in the ABNF production rule for 'body-stmts' (Section 14 in RFC 7950). The argument of the 'md:annotation' statement defines the name of the annotation. Syntactically, it is a YANG identifier as defined in Section 6.2 of RFC 7950. An annotation defined with this 'extension' statement inherits the namespace and other context from the YANG module in which it is defined. The data type of the annotation value is specified in the same way as for a leaf data node using the 'type' statement. The semantics of the annotation and other documentation can be specified using the following standard YANG substatements (all are optional): 'description', 'if-feature', 'reference', 'status', and 'units'. A server announces support for a particular annotation by including the module in which the annotation is defined among the advertised YANG modules, e.g., in a NETCONF <hello> message or in the YANG library (RFC 7950). The annotation can then be attached to any instance of a data node defined in any YANG module that is advertised by the server. XML encoding and JSON encoding of annotations are defined in RFC 7952."; } } <CODE ENDS>

This document registers a URI in the "IETF XML Registry" . Following the format in RFC 3688, the following registration has been made.

--------------------------------------------------------------------- URI: urn:ietf:params:xml:ns:yang:ietf-yang-metadata Registrant Contact: The NETMOD WG of the IETF. XML: N/A, the requested URI is an XML namespace. ---------------------------------------------------------------------

This document registers a YANG module in the "YANG Module Names" registry .

--------------------------------------------------------------------- Name: ietf-yang-metadata Namespace: urn:ietf:params:xml:ns:yang:ietf-yang-metadata Prefix: md Reference: RFC 7952 ---------------------------------------------------------------------

This document introduces a mechanism for defining metadata annotations in YANG modules and attaching them to instances of YANG data nodes. By itself, this mechanism represents no security threat. Security implications of a particular annotation defined using this mechanism MUST be duly considered and documented in the annotation's definition. An annotation SHOULD be subject to the same or stricter access control rules as the data node instance to which the annotation is attached. It is RECOMMENDED that security-sensitive or privacy-sensitive data be modeled as regular YANG data nodes rather than annotations.

International Organization for Standardization International Organization for Standardization International Organization for Standardization International Organization for Standardization

The author wishes to thank Andy Bierman, Martin Bjorklund, Benoit Claise, Juergen Schoenwaelder, and Kent Watsen for their helpful comments and suggestions.